1. Field of the Invention
The present invention relates to a wire cutting device of a wire electric discharge machine, which, in the wire electric discharge machine, applies a current through a wire and applies tension to fuse the wire.
2. Description of the Prior Art
A wire electric discharge machine provides a machining start hole in a workpiece beforehand, inserts a wire into the machining start hole, and thereafter machines the workpiece by electric discharge. Particularly, when machining a plurality of workpieces, or when taking out a plurality of machining objects from one workpiece in continuous unattended operation, when one work is finished, a wire is cut, then a wire is automatically inserted into the next machining start hole for the next machining to be automatically connected, and the next machining is performed. Therefore, a highly reliable automatic wire cutting/connection function is absolutely necessary. This function is for automatically cutting a wire after machining which is started from a certain machining start hole is finished, moving a wire guide to the next machining start hole, then automatically connecting the wire in that position to which the wire guide was moved, and then starting the next machining. Speeding up automatic wire cutting/connection operation is an important development subject for reducing machining time.
As a conventionally proposed method for automatically cutting a wire, there is a method in which a current is supplied to a wire, the wire is then heated by means of its own resistance and is at the same time tensioned, whereby the wire is fused by heat (see, for example, Japanese Patent Application Laid-open No. 2-53528). According to this method, in comparison with a mechanical cutting method where a cutter or the like is used, an end of the wire can be formed into a pointy shape, so the wire can be smoothly led through a wire guide and workpiece at the time of wire connection. Therefore, speedy and secure wire connection is possible.
In order to reduce time to cut a wire, the time until the temperature at which a wire can be cut is reached may be reduced by increasing a current to be supplied to the wire so as to increase the heat value of the wire. However, if a current is increased too much, an end of the wire turns into a spherical shape, so the wire cannot be inserted smoothly, and it takes time for wire connection. Moreover, wire connection may be failed. As described in Japanese Patent Application Laid-open No. 2-237724, the shape of an end of a wire changes according to values of a current and tension applied to the wire, thus, in order to realize the shape of wire that is suitable for automatic wire connection, it is necessary to select an optimum value of a current and of tension in accordance with the material and wire diameter.
Also, there is proposed a method in which a wire is held for a predetermined amount of time while being applied with a predetermined of tension and a fusing current, for the purpose of forming an end portion of the wire into a pointy shape without damaging an object to be machined or a part such as a wire guide (see, for example, Japanese Patent Application Laid-open No. 5-305522). Here, “a predetermined time” indicates time until the wire is fused under a condition where a fusing current is applied and then the end portion of the wire is formed into a spherical shape (an optimum value of tension and an optimum value of a fusing current).
In this manner, a value of a current supplied to a wire is determined itself, resulting in a problem where time for cutting the wire cannot be reduced. It should be noted that a method for changing a value of a current supplied to a wire during fusing operation is described in Japanese Patent Application Laid-open No. 5-220624 and Japanese Patent Application Laid-open No. 4-331022. Japanese Patent Application Laid-open No. 5-220624 describes increases of the amount of current with the course of time during fusing operation will be able to save determining, before starting the fusing operation, a value of the supplied current in accordance with the type or wire diameter of the wire. Further, Japanese Patent Application Laid-open No. 4-331022 describes how a wire is securely fused regardless of variation in values of resistance within a disconnecting circuit, by increasing the amount of a current after a predetermined amount of time since the cutting has started.